The objective of our work is to delineate the physico-chemical aspects of hormone-receptor interaction by means of fluorescence polarization kinetic and equilibrium measurements utilizing purified receptor preparations, subcellular fractions and whole cells under a variety of conditions of differing concentrations of the hormone, competitors, inhibitors, chaotropic ions, solvents and pH. Kinetic methods (static stopped-flow, temperature jump and dilution jump) in conjunction with equilibrium measurements create a penetrating approach for segregating the mechanistic details of complicated biological reactions. The fluorescence polarization approach has several key features which make it a valuable tool for this area. First, the sensitivity is as high as that of radiolabeling, while at the same time the fluorescence technique does not require any separation step to determine the bound/free ratio in an experiment. As a result reactions can be monitored with high time resolution and without disturbing them in any way. Direct visualization of the interaction between whole cells or tissue sections and fluorescent hormones can be accomplished by means of fluorescence microscopy which now is feasible down to the 10 minus to the 10th power range of hormone concentration.